The selection of semiconductor materials in the manufacture of semiconductor devices typically involves consideration of material properties such as, e.g., conductivity, dopant concentration and distribution, dopant activation energy, trap concentrations and distributions, trap depths, and trap cross sections. While experimental determination of such properties is particularly important at the design and development stage of devices, similar measurements may be of interest at various stages of a manufacturing process, e.g., upon implantation or diffusion of dopants.
Prominent among methods for determining electrical properties of matter are methods which involve capacitance measurement upon application of an electrical signal. For example, U.S. Pat. No. 3,315,156 discloses a method for determining resistance of a disc of pure semiconductor material by an arrangement in which a semiconductor sample is placed between electrodes. A radiofrequency signal is applied and distance between electrodes is adjusted until resonance is realized in the circuit.
While capacitance methods are applicable most directly to the measurement of dielectric properties of high-resistivity materials, they can be adapted to the measurement of properties of doped semiconductor materials by using a reverse biased p-n junction structure as disclosed in the book by A. G. Milnes, "Deep Impurities in Semiconductors", Wiley, 1973, p. 215. By employing doped semiconductor materials in this fashion, conductivity is reduced to a level at which capacitance measurement is practicable, e.g., by means of a radio frequency bridge as disclosed in the paper by G. L. Miller et al., "Capacitance Transient Spectroscopy", Ann. Rev. Mater. Sci. 1977, pages 377-448.
The determination of certain semiconductor properties such as, e.g., impurity concentrations is facilitated by capacitance measurement as a function of sample temperature. Such a technique is disclosed in the paper by D. L. Loose, "Admittance Spectroscopy of Deep Impurity Levels: ZnTe Schottky Barriers", Applied Physics Letters Vol. 21, No. 2, 15 July 1972, pages 54-56, where admittance data as a function of frequency and temperature are used to gain information regarding types and concentrations of deep impurity levels.
Dielectric properties of a material may be determined by measuring current flow induced either by a sinusoidal voltage or else by a voltage pulse. These regimes are known as steady state and transient measuring systems, respectively. Moreover, in order to gain full spatial information about defects, the properties of a sample can be monitored as a function of a localized perturbation such as produced, e.g., by a scanning electron or laser beam. This approach is described in the paper by P. M. Petroff et al., "A New Spectroscopic Technique for Imaging the Spatial Distribution of Non-Radiative Defects in a Scanning Transmission Electron Microscope", Applied Physics, Letters, Vol. 31, No. 2, 15 July 1977, pages 60-62.
Attendant primarily to scanning beam imaging methods is a concern with the magnitude of the signal produced when only a relatively small portion of a semiconductor sample is perturbed at any one time. Specifically, in the interest of image resolution, a small spot size is desirable, but, in the interest of adequate signal-to-noise ratio, a certain minimal spot size is required to produce a meaningful signal. This concern motivates the search for imaging methods having enhanced signal-to-noise ratio.